1. Field of the Invention
The present invention relates to an insulating substrate composed of insulative ceramic layers having a proper breakdown voltage, a method of manufacturing such an insulating substrate, and a semiconductor device employing the insulating substrate. The present invention also relates to a module semiconductor device such as a power semiconductor device having semiconductor chips to control large current.
2. Description of the Prior Art
Semiconductor chips used to control a small current of several milliamperes to several amperes. Presently, they are able to control a large current of several tens of amperes to about 100 amperes. There are module semiconductor devices that incorporate semiconductor chips in an insulative resin case to control a current of several hundreds of amperes to about 1000 amperes. The module semiconductor devices are widely used as power sources to drive vehicles or large motors in rolling plants and chemical plants.
The module semiconductor devices are capable of not only handling large current but also providing a high breakdown voltage of, for example, 5 kV. In the future, a breakdown voltage of 10 kV or higher will be required. A higher current value means higher heat generation. The module semiconductor devices must efficiently dissipate heat from semiconductor chips, and for this, they must be made of material of high thermal conductivity.
FIG. 1B is a sectional view showing a module semiconductor device 65 according to a prior art. Semiconductor chips 57 are joined to the top surface of an insulating substrate 51 with a solder layer 59. The bottom surface of the insulating substrate 51 is joined to the top surface of a base 60 with a solder layer 61. The base 60 is made of metal or a composite material of metal and ceramics. The semiconductor chips 57, solder layers 59 and 61, and insulating substrate 51 are sealed with insulative sealing resin 63 and are packed in an insulative resin case 64, to form the module semiconductor device 65. A water- or air-cooled heat sink 66 is fixed to the bottom surface of the base 60 with bolts 67.
FIG. 1A shows the insulating substrate 51 of the module semiconductor device 65 of FIG. 1B. The insulating substrate 51 consists of an insulative ceramic layer 52 and conductive layers 55 and 56. The conductive layers 55 and 56 are joined to the top and bottom surfaces of the ceramic layer 52, respectively, by direct bonding copper method or active metal brazing method.
The module semiconductor device 65 of the prior art dissipates heat from the semiconductor chips 57 to the insulating substrate 51, base 60, and heat sink 66. Therefore, the insulating substrate 51, in particular, the conductive layers 55 and 56 must have good thermal conductivity. For this, the conductive layers 55 and 56 are usually made of copper, aluminum, an alloy thereof, or a composite material thereof.
A breakdown voltage of the module semiconductor device 65 is determined by that of the semiconductor chips 57, which is determined by that of the insulating substrate 51. To improve the breakdown voltage of the module semiconductor device 65, it is necessary to improve the breakdown voltage of the insulating substrate 51. Improving the breakdown voltage of the insulating substrate 51 is achievable by thickening the ceramic layer 52. The ceramic layer 52 may be made of aluminum oxide (Al2O3) or aluminum nitride (AlN) having a good dielectric property.
A module semiconductor device has a layered structure of semiconductor chips and an insulative ceramic layer that have low thermal expansion coefficients, and conductive layers and a base that have high thermal expansion coefficients. When the semiconductor chips are energized, they generate heat to repeatedly apply large thermal stress onto these elements and sometimes crack the ceramic layer to cause a dielectric breakdown.
To cope with this problem, Japanese Unexamined Patent Publication No. 9-275166 forms a layer of refractory metal such as tungsten (W) and molybdenum (Mo) whose thermal expansion coefficients are close to that of an insulative ceramic layer of an insulating substrate, on each of the top and bottom surfaces of the ceramic layer, to relax thermal stress on the ceramic layer and reinforce the same. The refractory metal, however, has lower thermal conductivity than copper and aluminum, and therefore, is not always preferable in terms of cooling semiconductor chips. In addition, the conventional copper and aluminum plastically deform to relax thermal stress on an insulative ceramic layer. On the other hand, the refractory metal has a very high elastic coefficient and yield strength, and therefore, provides no stress relaxing effect. An analysis of thermal stress on refractory metal layers shows that high thermal stress occurs on the refractory metal layers. In addition, the fracture toughness of the refractory metal is not high. Due to these factors, the refractory metal layers have a high possibility of causing cracks due to thermal stress.
Japanese Unexamined Patent Publication Nos. 8-195450 and 8-195458 employ aluminum oxide to form an insulative ceramic layer to prevent cracks. Aluminum oxide may be stronger than aluminum nitride but has lower thermal conductivity than the aluminum nitride. This low thermal conductivity of aluminum oxide may further drop if reinforcing elements are added to aluminum oxide.
Materials used to form insulative ceramic layers generally have low fracture toughness and high crack sensitivity. Even a fine defect on the surface of an insulative ceramic layer may start a crack running across the thickness thereof. The inventors of the present invention studied the details of breaking behavior of insulating substrates through thermal cycles and found that the fracture toughness of insulative ceramic materials is very low compared with that of metal materials, and once a crack occurs on a layer made of an insulative ceramic material, it quickly propagates across the thickness of the layer. The insulative ceramic materials have a breakdown voltage of 10 kV or above per a thickness of 1-mm. However, even a fine crack across the 1-mm thickness deteriorates the breakdown voltage to that of air, i.e., about 3 to 4 kV. This may instantaneously cause a dielectric breakdown of a module semiconductor device that employs the ceramic layer. In high humidity, the breakdown voltage of air further deteriorates to cause a dielectric breakdown at a voltage lower than 3 kV or 4 kV.
If an insulative ceramic layer of 1-mm thick has no cracks running across the thickness thereof, it will maintain a breakdown voltage of 10 kV or higher. It is important, therefore, to prevent cracks on insulative ceramic layers.
Ceramic materials have individual strength values that widely vary from material to material. Accordingly, strength test data for a given ceramic material must statistically be processed with the use of standard deviations and Weibull distributions before determining a stress threshold for the ceramic material. Once the stress threshold is determined, it is used to design a module semiconductor device that employs the ceramic material.
Among many insulative ceramic layers, some may have strength that is below design strength. To prevent a dielectric breakdown of module semiconductor devices that are made from such ceramic layers, it is necessary to completely eliminate cracks from the ceramic layers. To achieve this, design stress for the ceramic layers must be set as small as possible. This, however, is impractical to achieve. In this way, ceramic materials have a reliability problem.